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Rhizoma peanut (Arachis glabrata Benth.; RP) is an important perennial forage legume in the US Gulf Coast region, but it is vegetatively propagated from rhizomes and relatively slow to establish. There are reports that N fertilization enhances RP establishment, but other evidence suggests plant N content and establishment rate may not benefit. Understanding the effect of N fertilizer on RP biological N2 fixation will help elucidate this response. Our objective was to determine the effect of RP genotype and N fertilization during shoot emergence on N2 fixation and N accumulation during the year of planting. Replicated field experiments were conducted in adjoining fields; one was planted in 2016 and the other in 2017. Treatments were the factorial combinations of two RP entries (decumbent germplasm Ecoturf and upright cultivar 'UF Tito') and three N rates (0, 40, and 80 kg N ha−1 applied 10 wk after planting; 1% of total N as 15N) arranged in a randomized complete block design with four replications. Across N rates, upright cultivar UF Tito generally had greater N concentration and content in aboveground and belowground plant parts compared with decumbent germplasm Ecoturf. Nitrogen fertilization generally reduced the percentage of N derived from atmosphere and the amount of fixed N (BNF) during the year of planting, but it did not affect overall N concentration or content. These data suggest RP increased soil N uptake to compensate for reduced BNF following N fertilization, indicating starter N application had no measurable benefit for legume establishment under these conditions. [ABSTRACT FROM AUTHOR]

Introduction and aims: In the context of increasing population and decreasing soil fertility, food security is one of humanity's greatest challenges. Large amounts of waste, such as sewage sludge, are produced annually, with their final disposal causing environmental pollution and hazards to human health. Sludge has high amounts of nitrogen (N), and, when safely recycled by applying it into the soil as composted sewage sludge (CSS), its residual effect may provide gradual N release to crops. A field study was conducted in the Brazilian Cerrado. The aims were to investigate the residual effect of successive applications of CSS as a source of N in the common bean (Phaseolus vulgaris L. cv. BRS Estilo)-palisade grass (Urochloa brizantha (A.Rich.) R.D. Webster)-soybean (Glycine max L.) rotation under notillage. Additionally, N cycling was monitored through changes in N metabolism; the efficiency of biological N2 fixation (BNF) and its implications for plant nutrition, development, and productivity, was also assessed. Methods: The experiment consisted of a randomized complete block design comparing four CSS rates (10, 15, 20, and 25 Mg ha-1, wet basis) to a control treatment (without adding mineral or organic fertilizer) over two crop years. Multiple plant and soil analyses (plant development and crop yield, Falker chlorophyll index (FCI), enzymatic, biochemical, 15N natural abundance, was evaluated, root and shoot N accumulation, etc.) were evaluated. Results and discussion: Results showed that CSS: i) maintained adequate N levels for all crops, increasing their productivity; ii) promoted efficient BNF, due to the stability of ureide metabolism in plants and increased protein content; iii) increased the nitrate content and the nitrate reductase activity in soybean; iv) affected urease activity and ammonium content due to changes in the plant's urea metabolism; v) increased N accumulation in the aerial part of palisade grass. Composted sewage sludge can be used as an alternative source to meet crops' N requirements, promoting productivity gains and N cycling through forage and improving N metabolism. [ABSTRACT FROM AUTHOR]

Asymbiotic N2 fixation (ANF) is a major nitrogen (N) input pathway to terrestrial ecosystems. However, there is considerable uncertainty about how lithology affects ANF. Here, we measured ANF rates in soil, litter, and moss in forests underlain by limestone (limestone forest), dolomite (dolomite forest), and clasolite (clasolite forest), respectively, in southwest China. Effects of lithology on soil ANF rate varied seasonally. The rate was highest in the dolomite forest during the wet season, but was highest in the limestone forest during the dry season. The overall soil ANF rate was significantly higher in the limestone forest than in the clasolite forest. Litter ANF rate was significantly higher in the limestone forest and dolomite forest than in the clasolite forest regardless of season. There was no significant difference in moss ANF rate among the three types of forests in both seasons. The annual N2 fixation rate was highest in the limestone forest (1.72 ± 0.27 kg N ha−1 y−1) but lowest in the clasolite forest (0.70 ± 0.08 kg N ha−1 y−1). In the wet season, the variation of ANF rates was best explained by soil nitrate and available iron for soil, and by litter N and calcium content for litter. In the dry season, the variation of ANF rates was best explained by soil water content, ammonia and total phosphorus for soil, and by litter water content and calcium content for litter. No strong explanatory variables were identified for ANF in moss during the wet or dry season. Our findings suggest that lithology significantly affected ANF in soil and litter, but not in moss, and hence should be considered in Earth system models to facilitate better prediction of N inputs via biological N2 fixation under global change. [ABSTRACT FROM AUTHOR]

Biological N2 fixation, a major pathway for new nitrogen (N) input to terrestrial ecosystems, largely determines the dynamics of ecosystem structure and functions under global change. Nevertheless, the responses of N2 fixation to multiple global change factors remain poorly understood. Here, saplings of two N2-fixing plant species, Alnus cremastogyne and Cajanus cajan, were grown at rural and urban sites, respectively, with the latter representing an environment with changes in multiple factors occurring simultaneously. Symbiotic N2 fixation per unit of nodule was significantly higher at the urban site than the rural site for A. cremastogyne, but the rates were comparable between the two sites for C. cajan. The nodule investments were significantly lower at the urban site relative to the rural site for both species. Symbiotic N2 fixation per plant increased by 31.2 times for A. cremastogyne, while that decreased by 88.2% for C. cajan at the urban site compared to the rural site. Asymbiotic N2 fixation rate in soil decreased by 46.2% at the urban site relative to the rural site. The decrease in symbiotic N2 fixation per plant for C. cajan and asymbiotic N2 fixation in soil was probably attributed to higher N deposition under the urban conditions, while the increase in symbiotic N2 fixation per plant for A. cremastogyne was probably related to the higher levels of temperature, atmospheric CO2, and phosphorus deposition at the urban site. The responses of N2 fixation to multiple global change factors and the underlying mechanisms may be divergent either between symbiotic and asymbiotic forms or among N2-fixing plant species. While causative evidence is urgently needed, we argue that these differences should be considered in Earth system models to improve the prediction of N2 fixation under global change.

Introduction and aimsIn the context of increasing population and decreasing soil fertility, food security is one of humanity’s greatest challenges. Large amounts of waste, such as sewage sludge, are produced annually, with their final disposal causing environmental pollution and hazards to human health. Sludge has high amounts of nitrogen (N), and, when safely recycled by applying it into the soil as composted sewage sludge (CSS), its residual effect may provide gradual N release to crops. A field study was conducted in the Brazilian Cerrado. The aims were to investigate the residual effect of successive applications of CSS as a source of N in the common bean (Phaseolus vulgaris L. cv. BRS Estilo)-palisade grass (Urochloa brizantha (A.Rich.) R.D. Webster)-soybean (Glycine max L.) rotation under no-tillage. Additionally, N cycling was monitored through changes in N metabolism; the efficiency of biological N2 fixation (BNF) and its implications for plant nutrition, development, and productivity, was also assessed.MethodsThe experiment consisted of a randomized complete block design comparing four CSS rates (10, 15, 20, and 25 Mg ha-1, wet basis) to a control treatment (without adding mineral or organic fertilizer) over two crop years. Multiple plant and soil analyses (plant development and crop yield, Falker chlorophyll index (FCI), enzymatic, biochemical, 15N natural abundance, was evaluated, root and shoot N accumulation, etc.) were evaluated.Results and discussionResults showed that CSS: i) maintained adequate N levels for all crops, increasing their productivity; ii) promoted efficient BNF, due to the stability of ureide metabolism in plants and increased protein content; iii) increased the nitrate content and the nitrate reductase activity in soybean; iv) affected urease activity and ammonium content due to changes in the plant’s urea metabolism; v) increased N accumulation in the aerial part of palisade grass. Composted sewage sludge can be used as an alternative source to meet crops’ N requirements, promoting productivity gains and N cycling through forage and improving N metabolism.

Legume cover crops in temperate cropping systems can fix substantial amounts of nitrogen (N) and reduce N fertiliser requirements for subsequent crops. However, little is known about potential biological N2 fixation by summer cover crop legumes in the short summer fallow in Mediterranean-type cropping systems. Six legume species (balansa clover, barrel medic, mung bean, sunn hemp, lablab and cowpea) were grown for 8–9 weeks in the field in semi-arid southern Australia during the summer fallow, and in a glasshouse experiment, to estimate N2 fixation using the 15N natural abundance method. Cowpea, sunn hemp and lablab produced 1.2–3.0 t ha−1 biomass in the field while balansa clover and barrel medic produced < 1.0 t ha−1. The percent of N derived from the atmosphere (%Ndfa) in the field ranged from 39% in barrel medic to 73% in sunn hemp, but only 15% (balansa clover) to 33% (sunn hemp) in the glasshouse experiment, likely due to higher soil mineral N availability in the glasshouse study. Biological N2 fixation of cowpea and sunn hemp in the field was 46–55 kg N ha−1, while N2 fixation in lablab and mung bean was lower (around 26 kg N ha−1). The N2 fixation in cowpea and sunn hemp of around 50 kg N ha−1 with supplementary irrigation in the field trial likely represents the upper limit of N contributions in the field in typically hot, dry summer conditions in Mediterranean-type climates. Given that any increase in summer cover crop biomass will have implications for water balances and subsequent cash crop growth, maximising N benefits of legume cover crops will rely on increasing the %Ndfa through improved rhizobium strains or inoculation technologies. This study provides the first known estimates of biological N2 fixation by legume cover crops in the summer fallow period in cropping systems in Mediterranean-type environments, providing a benchmark for further studies.

Biological N2 fixation (BNF) is a major pathway of external nitrogen input to terrestrial ecosystems. Nevertheless, the relative effects of diazotrophic community and soil properties on symbiotic (SNF) or free-living (FNF) N2 fixation remain poorly understood. Here, 20 sites of kudzu (Pueraria lobata) communities, which were at the early succession stage following agricultural abandonment, were selected across a karst area in southwest China. BNF rates were determined using the acetylene reduction assay calibrated with the 15N2 fixation method. The average rate was 17.81 ± 2.17 mg N g−1 day−1 nodule for SNF and 45.36 ± 5.15 ng N g−1 day−1 soil for FNF. Diazotrophic communities were dominated by Bradyrhizobium at the genus level in both nodule and soil. According to structural equation modeling, the relative abundances of two diazotrophic genera (Desulfovibrio and Geobacter) were the strongest explanatory variable for the variation of SNF rates with the second strongest variable being nifH gene abundance. Soil water content and available phosphorus indirectly affected SNF via their effects on diazotrophic community composition. In contrast, soil FNF rates were most pronouncedly affected by the availability of vanadium and iron, followed by soil water content, available phosphorus, and the relative abundances of two diazotrophic genera, i.e., Burkholderia and Cupriavidus. Our findings therefore help to improve the understanding of the relationship between BNF rates and their associated diazotrophic communities, and hence benefit a better prediction of BNF under global change. [ABSTRACT FROM AUTHOR]

Biological N2 fixation, a major pathway for new nitrogen (N) input to terrestrial ecosystems, largely determines the dynamics of ecosystem structure and functions under global change. Nevertheless, the responses of N2 fixation to multiple global change factors remain poorly understood. Here, saplings of two N2-fixing plant species, Alnus cremastogyne and Cajanus cajan, were grown at rural and urban sites, respectively, with the latter representing an environment with changes in multiple factors occurring simultaneously. Symbiotic N2 fixation per unit of nodule was significantly higher at the urban site than the rural site for A. cremastogyne, but the rates were comparable between the two sites for C. cajan. The nodule investments were significantly lower at the urban site relative to the rural site for both species. Symbiotic N2 fixation per plant increased by 31.2 times for A. cremastogyne, while that decreased by 88.2% for C. cajan at the urban site compared to the rural site. Asymbiotic N2 fixation rate in soil decreased by 46.2% at the urban site relative to the rural site. The decrease in symbiotic N2 fixation per plant for C. cajan and asymbiotic N2 fixation in soil was probably attributed to higher N deposition under the urban conditions, while the increase in symbiotic N2 fixation per plant for A. cremastogyne was probably related to the higher levels of temperature, atmospheric CO2, and phosphorus deposition at the urban site. The responses of N2 fixation to multiple global change factors and the underlying mechanisms may be divergent either between symbiotic and asymbiotic forms or among N2-fixing plant species. While causative evidence is urgently needed, we argue that these differences should be considered in Earth system models to improve the prediction of N2 fixation under global change. [ABSTRACT FROM AUTHOR]

N2 fixation is essential to the sustainability and operation of nitrogen systems but is energetically expensive. We developed a model and used sensitivity analysis to identify the impact of aerobic and anaerobic waste digestion, crop harvest index, rates of recovery of recalcitrant N, and the rate of N2 fixation in a system combining nitrogen fixation and recycling. The model indicates that the rate of N2 fixation, loss from reactors, fertilization efficiency, and crop harvest index have the largest impact on maintaining bioavailable N. N recoveries from aerobic and anaerobic digestion, as well as direct-to-soil fertilization, are not well characterized, but the case studies using this model indicate that their efficiencies are critical to N recovery. The findings of this model and its presented case studies can be used as a guide in the design of closed-loop habitats both on Earth and in space. These results reveal a clear need for continued research in the areas of N-efficient digestion, fertilization, and fixation.

Introduction: Biological N2 fixation in feather-mosses is one of the largest inputs of new nitrogen (N) to boreal forest ecosystems; however, revealing the fate of newly fixed N within the bryosphere (i.e. bryophytes and their associated organisms) remains uncertain. Methods: Herein, we combined 15N tracers, high resolution secondary ion mass-spectrometry (NanoSIMS) and a molecular survey of bacterial, fungal and diazotrophic communities, to determine the origin and transfer pathways of newly fixed N2 within feather-moss (Pleurozium schreberi) and its associated microbiome. Results: NanoSIMS images reveal that newly fixed N2, derived from cyanobacteria, is incorporated into moss tissues and associated bacteria, fungi and micro-algae. Discussion: These images demonstrate that previous assumptions that newly fixed N2 is sequestered into moss tissue and only released by decomposition are not correct. We provide the first empirical evidence of new pathways for N2 fixed in feather-mosses to enter the boreal forest ecosystem (i.e. through its microbiome) and discuss the implications for wider ecosystem function. [ABSTRACT FROM AUTHOR]

Legume cover crops in temperate cropping systems can fix substantial amounts of nitrogen (N) and reduce N fertiliser requirements for subsequent crops. However, little is known about potential biological N2 fixation by summer cover crop legumes in the short summer fallow in Mediterranean-type cropping systems. Six legume species (balansa clover, barrel medic, mung bean, sunn hemp, lablab and cowpea) were grown for 8–9 weeks in the field in semi-arid southern Australia during the summer fallow, and in a glasshouse experiment, to estimate N2 fixation using the 15N natural abundance method. Cowpea, sunn hemp and lablab produced 1.2–3.0 t ha−1 biomass in the field while balansa clover and barrel medic produced < 1.0 t ha−1. The percent of N derived from the atmosphere (%Ndfa) in the field ranged from 39% in barrel medic to 73% in sunn hemp, but only 15% (balansa clover) to 33% (sunn hemp) in the glasshouse experiment, likely due to higher soil mineral N availability in the glasshouse study. Biological N2 fixation of cowpea and sunn hemp in the field was 46–55 kg N ha−1, while N2 fixation in lablab and mung bean was lower (around 26 kg N ha−1). The N2 fixation in cowpea and sunn hemp of around 50 kg N ha−1 with supplementary irrigation in the field trial likely represents the upper limit of N contributions in the field in typically hot, dry summer conditions in Mediterranean-type climates. Given that any increase in summer cover crop biomass will have implications for water balances and subsequent cash crop growth, maximising N benefits of legume cover crops will rely on increasing the %Ndfa through improved rhizobium strains or inoculation technologies. This study provides the first known estimates of biological N2 fixation by legume cover crops in the summer fallow period in cropping systems in Mediterranean-type environments, providing a benchmark for further studies. [ABSTRACT FROM AUTHOR]

IntroductionBiological N2 fixation in feather-mosses is one of the largest inputs of new nitrogen (N) to boreal forest ecosystems; however, revealing the fate of newly fixed N within the bryosphere (i.e. bryophytes and their associated organisms) remains uncertain.MethodsHerein, we combined 15N tracers, high resolution secondary ion mass-spectrometry (NanoSIMS) and a molecular survey of bacterial, fungal and diazotrophic communities, to determine the origin and transfer pathways of newly fixed N2 within feather-moss (Pleurozium schreberi) and its associated microbiome.ResultsNanoSIMS images reveal that newly fixed N2, derived from cyanobacteria, is incorporated into moss tissues and associated bacteria, fungi and micro-algae. DiscussionThese images demonstrate that previous assumptions that newly fixed N2 is sequestered into moss tissue and only released by decomposition are not correct. We provide the first empirical evidence of new pathways for N2 fixed in feather-mosses to enter the boreal forest ecosystem (i.e. through its microbiome) and discuss the implications for wider ecosystem function.

Nitrogen (N) resorption from senescing leaves enables plants to reuse N, making them less dependent on current N uptake from the environment, leading to higher fitness, particularly under low N supply. Species that form a symbiotic association with N2-fixing bacteria have not evolved proficient N resorption, i.e., they retain more N in the senesced leaves than non-N2-fixing species. However, the physiological mechanism underlying the difference is still unknown. Metabolic and structural protein contents in green and senesced leaves, as well as protein degradation during leaf senescence—a critical initial process for subsequent N resorption—were determined in four N2-fixing legumes and in four non-N2-fixers. The metabolic proteins were highly degraded in legumes and to a lesser extent in nonlegumes. Nonetheless, legumes retained more metabolic proteins in their senesced leaves than nonlegumes, because symbiotic N2 fixation improved the metabolic protein content in green leaves. Symbiotic N2 fixation did not change the structural protein content in green leaves. The structural proteins were moderately degraded in nonlegumes, and almost undegraded in legumes, and more structural proteins remained in the senesced leaves of legumes than in those of nonlegumes. The higher metabolic and structural protein contents in the senesced leaves of N2-fixing legumes properly explained the less proficient N resorption. This is an important step in unraveling molecular mechanisms of different N conservation strategies among plant functional types. [ABSTRACT FROM AUTHOR]

Commercial inoculants are often used to inoculate field-grown soybean in Europe. However, nodulation efficiencies in these areas are often low. To enhance biological nitrogen (N) fixation and increase domestic legume production, indigenous strains that are adapted to local conditions could be used to develop more effective inoculants. The objective of this study was to assess the ability of locally isolated Bradyrhizobium strains to enhance soybean productivity in different growing conditions of Northeast Germany. Three indigenous Bradyrhizobium isolates (GMF14, GMM36, and GEM96) were tested in combination with different soybean cultivars of different maturity groups and quality characteristics in one field trial and two greenhouse studies. The results showed a highly significant strain × cultivar interactions on nodulation response. Independent of the Bradyrhizobium strain, inoculated plants in the greenhouse showed higher nodulation, which corresponded with an increased N uptake than that in field conditions. There were significantly higher nodule numbers and nodule dry weights following GMF14 and GMM36 inoculation in well-watered soil, but only minor differences under drought conditions. Inoculation of the soybean cultivar Merlin with the strain GEM96 enhanced nodulation but did not correspond to an increased grain yield under field conditions. USDA110 was consistent in improving the grain yield of soybean cultivars Sultana and Siroca. On the other hand, GMM36 inoculation to Sultana and GEM96 inoculation to Siroca resulted in similar yields. Our results demonstrate that inoculation of locally adapted soybean cultivars with the indigenous isolates improves nodulation and yield attributes. Thus, to attain optimal symbiotic performance, the strains need to be matched with specific cultivars. [ABSTRACT FROM AUTHOR]

Commercial inoculants are often used to inoculate field-grown soybean in Europe. However, nodulation efficiencies in these areas are often low. To enhance biological nitrogen (N) fixation and increase domestic legume production, indigenous strains that are adapted to local conditions could be used to develop more effective inoculants. The objective of this study was to assess the ability of locally isolated Bradyrhizobium strains to enhance soybean productivity in different growing conditions of Northeast Germany. Three indigenous Bradyrhizobium isolates (GMF14, GMM36, and GEM96) were tested in combination with different soybean cultivars of different maturity groups and quality characteristics in one field trial and two greenhouse studies. The results showed a highly significant strain × cultivar interactions on nodulation response. Independent of the Bradyrhizobium strain, inoculated plants in the greenhouse showed higher nodulation, which corresponded with an increased N uptake than that in field conditions. There were significantly higher nodule numbers and nodule dry weights following GMF14 and GMM36 inoculation in well-watered soil, but only minor differences under drought conditions. Inoculation of the soybean cultivar Merlin with the strain GEM96 enhanced nodulation but did not correspond to an increased grain yield under field conditions. USDA110 was consistent in improving the grain yield of soybean cultivars Sultana and Siroca. On the other hand, GMM36 inoculation to Sultana and GEM96 inoculation to Siroca resulted in similar yields. Our results demonstrate that inoculation of locally adapted soybean cultivars with the indigenous isolates improves nodulation and yield attributes. Thus, to attain optimal symbiotic performance, the strains need to be matched with specific cultivars.

Background and aims: Organisms that are capable of nitrogen (N) fixation, such as some species of cyanobacteria in biological soil crusts (biocrusts), can be ecologically important especially in low-nutrient dryland ecosystems. In addition, woody plant encroachment is occurring in drylands globally, including many species of N2-fixing shrubs and trees. We determined the effects of nutrient deposition and shrub encroachment by Larrea tridentata on N2-fixation rates of biocrusts and compared these rates to another encroaching shrub, the N2-fixer Prosopis glandulosa in the northern Chihuahuan Desert. Methods: We used both the acetylene reduction assay (ARA) and the 15N natural abundance techniques to determine N2-fixation rates of biocrusts. We also assessed the effects of nutrient additions on biocrust N2-fixation rates using a long-term N addition experiment started in 2008 that is part of the Nutrient Network (NutNet). Results: There was no correlation between the ARA and δ15N methods. We found that P. glandulosa likely fixes more N (at least during the spring) than biocrusts based on δ15N content in roots. There was no significant difference in δ15N between P. glandulosa and our reference plant, L. tridentata. δ15N declined in N-fertilized plots presumably because biological nitrogen fixation (BNF) is costly or because added N has negative effects on microbial communities. Conclusions: Biocrusts fixed less nitrogen than N2-fixing legume shrubs during this sampling period. Nonetheless, biocrusts are still a significant contributor to N2-fixation in these nutrient-poor drylands based on the large proportion of soil surface area occupied by these organisms. [ABSTRACT FROM AUTHOR]

In grass-legume pastures, grazing management strategies are an essential factor affecting nitrogen (N) cycling. This study assessed the impact of grazing management on N cycling in rotationally-stocked mixed pastures of 'Marandu' palisadegrass (Brachiaria brizantha) and 'Comum' calopo (Calopogonium mucunoides). Treatments included three grazing management strategies, defined by interruption of the rest period when the canopy reached 90 (90LI), 95 (95LI) and 100% (100LI) of the interception of photosynthetically active radiation. A 2-yr experimental period was adopted. Plant litter responses, forage intake and livestock excretion were evaluated. No differences between grazing management were obtained for existing (294 g OM m−2) and deposited litter (6.7 g OM m−2 d−1, P > 0.10). Compared to the dry season, the litter decomposition rate increased 24.0%, and the half-life decreased 37.8% in the rainy season (P < 0.10). The N cycling via litter (553 g ha−1 d−1) was similar in all grazing management (P > 0.10). Less frequent defoliation (100LI) resulted in reduced proportion of legume intake (P < 0.10, 94.4 vs. 168.5 g kg−1; an average of 90LI and 95LI, respectively), lower N intake (123.1 vs. 194.1 g animal unit−1 d−1) and a lower input of N from biological fixation (73.2 vs. 97.8 kg ha−1 yr−1). Less frequent defoliation should be avoided because it reduces the N intake and N retained by animals, which caused a reduction in N utilisation efficiency by heifers. Thus, 95% light interception is the maximum limit to interrupt the regrowth in palisadegrass-calopo pastures. [ABSTRACT FROM AUTHOR]

Biological N2 fixation plays crucial roles in determining the pattern and pace of ecological succession and carbon sequestration. Nevertheless, the rates and controls of symbiotic (SNF) and free‐living (FNF) N2 fixation have rarely been investigated simultaneously. Here we measured SNF and soil FNF rates for eight early succession ecosystems dominated by kudzu (Pueraria lobata (Willd.) Ohwi) in a subtropical region of China in growing and nongrowing seasons. N2 fixation rates were measured using acetylene reduction assay against a 15N2 uptake method. Both SNF and soil FNF rates were significantly higher in the growing season than in the nongrowing season. The two forms of N2 fixation contributed equally to total N2 fixation rate across the eight sites with soil FNF being higher than SNF in half of the sites. Total N2 fixation rates ranged from 7.12 to 61.93 kg N ha−1 yr−1 with an average of 22.93 ± 6.52 kg N ha−1 yr−1. N2 fixation contributed significantly to soil nitrogen and carbon accumulation. Both SNF and FNF were significantly related to soil moisture. Additionally, SNF and soil FNF were strongly correlated with phosphorus availability and vanadium/iron availability, respectively, most likely implying that SNF was limited by phosphorus and soil FNF was limited by vanadium or iron. Our findings suggest that SNF and soil FNF may be limited by different sets of nutrients within an ecosystem. Plain Language Summary: Nitrogen input via biological N2 fixation largely controls the pattern and pace of ecological succession and carbon sequestration. However, there is uncertainty in terms of the rates and controls of symbiotic (SNF) and free‐living (FNF) N2 fixation. We investigated SNF and soil FNF rates for eight early succession ecosystems dominated by kudzu (Pueraria lobata (Willd.) Ohwi) in a subtropical region of China. We find that the two forms of N2 fixation contributed similar amounts of N to kudzu communities. SNF was likely limited by phosphorus, but soil FNF was likely limited by vanadium or iron. Our findings suggest that SNF and soil FNF may be limited by different sets of nutrients within an ecosystem. Key Points: Rates of and controls on N2 fixation in subtropical kudzu communities were studiedSymbiotic (SNF) and free‐living (FNF) N2 fixation contributed similar amounts of nitrogenControls on SNF and FNF were likely different [ABSTRACT FROM AUTHOR]

In theory, the phytoplankton community of freshwater ecosystems with low concentrations of dissolved inorganic nitrogen (DIN) can obtain this element by atmospheric nitrogen (N2) fixation. This process could explain the dominance of cyanobacteria in tropical reservoirs, yet is rarely quantified in these systems. Assessing the factors related to N2 fixation can assist in the management of cyanobacterial blooms. Our study characterized environmental factors related to N2 fixation in Brazilian tropical reservoirs with contrasting trophic states, and defined quantitative thresholds for water chemistry and physical characteristics that stimulated N2 fixation. We used field assays with 15N for estimating N2 fixation rates by phytoplankton. The highest rates normalized by chlorophyll‐a (maximum of 143 × 10−4 μg‐N μg‐Chl‐a−1 h−1) coincided with eutrophic conditions and presence of diazotrophs. Receiver operating characteristic (ROC) analysis provided significant thresholds for water temperature (≥22°C), soluble reactive phosphorus (SRP) (≥3.0 μg‐P L−1), total phosphorus (TP) (≥20.5 μg‐P L−1), DIN:SRP (≤487) and DIN:TP (≤82) molar ratios, chlorophyll‐a (≥12 μg L−1), and total suspended solids (≥4 mg L−1). Censored regressions confirmed that temperature, chlorophyll‐a, and phosphorus were important predictors of N2 fixation rates. In general, the N2 fixation rates determined in this study were lower than those found for temperate reservoirs. However, the temperature threshold of 22°C or above, identified in our analysis, suggests that phytoplankton have the potential to fix N2 throughout the year in tropical reservoirs. Our results suggested that phosphorus is the main nutrient controlling the rates of N2 fixation when N2‐fixing cyanobacteria were present. Phosphorus abatement is, thus, crucial for managing the trophic state and controlling N2‐fixing cyanobacteria in these ecosystems. [ABSTRACT FROM AUTHOR]

Abstract: The objective of this work was to evaluate the viability of liquid medium inoculation of Rhizobium etli in the planting furrow and to certify the efficiency of its strain UFLA 02-100 as a potential inoculant for common bean (Phaseolus vulgaris). The treatments consisted of three application rates of liquid inoculant applied in the planting furrows or in common bean seed, besides two controls without inoculation. The inoculant contained 109 cells of Rhizobium etli mL-1. Regardless of the application method, the yield obtained with the inoculation was equivalent to that of N from urea; however, the application in the furrows, at 0.6 L ha-1, is more advantageous due to the operational practicality and reduced costs.

Aims: This study was performed to determine if the contribution of biological N2 fixation (BNF) associated with Brazilian sugarcane cultivars could be increased by the inoculation with N2-fixing bacteria. Methods: The field experiment was planted with two sugarcane cultivars, inoculated or not, with five N2-fixing bacteria. All plant tissues, including belowground, were harvested for determination of dry matter, N accumulation and 15N abundance at five occasions until 450 days after planting. Results: Inoculation significantly increased total N accumulation in the aerial tissue of cultivar RB867515 from 147 to 199 kg N ha−1 and cultivar RB92579 from 126 to 192 N kg ha−1. At final harvest 15N abundances of entire sugarcane plants compared to that of plant-available N indicated that BNF inputs were over 64%. Total N derived from BNF and the soil was increased by inoculation, by 50 and 17 kg N ha−1 and 36 and 67 kg N ha−1, for the two cultivars respectively. Conclusions: Without inoculation, the two sugarcane cultivars obtained over 65% of N from BNF. Inoculation with the five component inoculant increased N accumulation from soil and BNF but with little impact on the proportion of N derived from BNF. [ABSTRACT FROM AUTHOR]

In this research, the effects of bacteria inoculation and iron application on the soybean plant, via pot experiments, using common soil series belonging to Aktas, Akkerpic, Akkuyu, Asagioba and Asi in the Amik Plain, were investigated. The experimental design consisted of two bacteria (BO: without inoculation and Bl: bacteria strain nr. 110), and two iron (FeO: without iron and Fel: 5 mg kg-1 Fe) on selected five soil series (SI: Aktas S2: Akkerpic; S3: Akkuyu; S4: Asagioba; S5: Asi) with three replications. Plant and soil samples were taken during the blooming period. Nodulation, biomass development and the nitrogen content were analyzed in plant samples, whereas the respiration (CO2 production), dehydrogenase enzyme activity (DHA) and microbial biomass carbon (MBC) contents of the soil have been determined. The number of nodules were 4, 12, 12, 4 and 0 number.plant-1 in Aktas, Akkerpic, Akkuyu, Asagi-oba and Asi, respectively. The nodule formation was not observed in non-inoculated pots. Within the inoculated applications, the plant was not developed in Asi series, therefore the nodule was not formed. In other soil series an average of five nodules per plant were determined. Bacterial inoculation and iron application together increased the nodule number up to 20 nodules per plant. The determined DHA, CO2 and MBC results were 110 μg TPF 10 gds1, 15.2 mg C 100 gds-1. 24 h-1 and 9.42 mg C 100 gds-1 in bacteria inoculated variant while the values in non-inoculated plants were 90 μg TPF 10 gds-1, 14.2 mg C 100 gds1, 24 h-1,7,93 mg C 100 gds-1, respectively. According to the general average results, bacterial inoculation and iron application have positive effects on nodule formation and nitrogen fixation in the common soil series of Amik Plain. [ABSTRACT FROM AUTHOR]

Modern rice cultivation relies on chemical nitrogen (N) fertilizer that ultimately degrades soil health and creates environmental pollution. The increasing concerns about environmental pollution, restoring soil health, and conservation of energy, public attention is now towards green manures as supplement for N fertilizer. Sesbania rostrata is a popular leguminous green manuring crop that fixes atmospheric N2, improves soil health, and rice productivity through incorporation of its biomass in to soil. About 40–60 days plants provide 5–6 tons dry biomass that can supplement 50–100% N requirement. In world, it is adapted as pre-rice or post-rice green manuring crop. Long-term use of S. rostrata improves soil health and increased rice yield by 9–11% over chemical fertilizer. Use of S. rostrata in large scale for rice production can reduce environmental pollution problems as it can cut down chemical N requirement and thus reduction dispersal of Nr to the environment. [ABSTRACT FROM AUTHOR]

Thermoleptolyngbya is a genus of non-heterocystous cyanobacteria that are typical inhabitants of hot spring microbial mats. These filamentous cyanobacteria are capable of nitrogen fixation. In this study, we examined the genome sequences of five publicly available Thermoleptolyngbya strains to explore their nitrogen fixation gene cluster. Analysis of the nitrogen-fixation clusters in these extremophilic strains revealed that the cluster is located in a single locus in Thermoleptolyngbyace. The average nucleotide and amino acid identities of the nitrogen-fixation cluster combined with phylogenetic reconstructions support that nitrogen fixation genes in Thermoleptolyngbyaceae are closely related to one another but also heterogeneous within the genus. The strains from Asia, and China more specifically, generate a separate clade within the genus. Among these strains Thermoleptolyngbya sp. PKUAC-SCTB121 has been selected for experimental validation of clade’s nitrogen fixation capacity. The acetylene reduction experiments of that strain shown that the strain can reduce acetylene to ethylene, indicating a fully functional nitrogenase. The activity of nitrogenase has been tested using different gas compositions across 72 h and exhibited a two-phase trend, high nitrogenase activity at the beginning of the assay that slowed down in the second phase of the analysis.

The Panicum maximum species (cv. Mombaça) is a forage plant well adapted to the different edaphoclimatic conditions found in Brazil; however, it requires high nitrogen fertilizer doses to ensure high yields. The present study aimed to assess the inoculation effect with associative diazotrophic bacteria strains on the yield and bromatological characteristics of the Guinea grass. A randomized block design was used, consisting of 25 treatments, being 23 of them inoculated with diazotrophic bacteria and two without inoculation (one with and the other without mineral nitrogen), with four repetitions. The analyzed variables were stem diameter; leaf width and length; leaf, stem and root dry mass yield; crude protein; acid and neutral detergent fibers. There were no differences among the treatments for root dry mass, stem diameter and acid detergent fiber. For the remaining variables, a positive effect of the inoculation with diazotrophic bacteria was observed, particularly for the following Unifenas strains: 100-06, 100-13, 100-26, 100-30, 10-35, 100-54, 100-69, 100-71 and 100-94. This reveals the potential of these bacterial strains for use at the sustainable production of the Guinea grass with a reduction in the use of nitrogen inputs.

Diazotrophic bacteria can reduce N2 into plant-available ammonium (NH4+), promoting plant growth and reducing nitrogen (N) fertilizer requirements. However, there are few systematic studies on the effects of diazotrophic bacteria on biological N2 fixation (BNF) contribution rate and host plant N uptake and metabolism. In this study, the interactions of the diazotrophic Paenibacillus beijingensis BJ-18 with wheat, maize, and cucumber were investigated when it was inoculated to these plant seedlings grown in both low N and high N soils, with un-inoculated plants as controls. This study showed that GFP-tagged P. beijingensis BJ-18 colonized inside and outside seedlings, forming rhizospheric and endophytic colonies in roots, stems, and leaves. The numbers of this bacterium in the inoculated plants depended on soil N levels. Under low N, inoculation significantly increased shoot dry weight (wheat 86.1%, maize 46.6%, and cucumber 103.6%) and root dry weight (wheat 46.0%, maize 47.5%, and cucumber 20.3%). The 15N-isotope-enrichment experiment indicated that plant seedlings derived 12.9–36.4% N from BNF. The transcript levels of nifH in the inoculated plants were 0.75–1.61 folds higher in low N soil than those in high N soil. Inoculation enhanced NH4+ and nitrate (NO3-) uptake from soil especially under low N. The total N in the inoculated plants were increased by 49.1–92.3% under low N and by 13–15.5% under high N. Inoculation enhanced activities of glutamine synthetase (GS) and nitrate reductase (NR) in plants, especially under low N. The expression levels of N uptake and N metabolism genes: AMT (ammonium transporter), NRT (nitrate transporter), NiR (nitrite reductase), NR, GS and GOGAT (glutamate synthase) in the inoculated plants grown under low N were up-regulated 1.5–91.9 folds, but they were not obviously changed under high N. Taken together, P. beijingensis BJ-18 was an effective, endophytic and diazotrophic bacterium. This bacterium contributed to plants with fixed N2, promoted plant growth and N uptake, and enhanced gene expression and enzyme activities involved in N uptake and assimilation in plants. However, these positive effects on plants were regulated by soil N status. This study might provide insight into the interactions of plants with beneficial associative and endophytic diazotrophic bacteria.

Evidence shows that nitrogen (N) addition may suppress or have no significant effect on asymbiotic N2 fixation (ANF) in soil, litter, or moss. However, the mechanisms underlying the differential responses of ANF to N addition are not well understood. The main objectives of the current study were to assess how ANF in soil, litter, and moss responds to N addition and whether topography modulates the responses of ANF in soil, litter, or moss to N addition. We conducted an N addition experiment at two topographic positions, that is, valley and slope of a secondary karst forest in southwest China. Three N addition treatments, that is, control (0 kg N·ha−1·year−1), moderate N addition (N50, 50 kg N·ha−1·year−1), and high N addition (N100, 100 kg N·ha−1·year−1) were included. Nitrogen addition had no significant effect on moss ANF at both topographic positions. Soil ANF was lower by 17.1% in the N100 plots relative to the control in the valley but was not significantly altered by N addition on the slope. In contrast, litter ANF was suppressed by N addition at both topographic positions by 77.9% to 87.4%. Our findings suggest that topography may modulate the responses of ANF to N addition, but the modulation effects likely differ among different ecosystem compartments. Plain Language Summary: Asymbiotic N2 fixation (ANF), a process carried out by free‐living N2‐fixing microorganisms to convert atmospheric N2 to ammonia, is an important source of new nitrogen for terrestrial ecosystems. Atmospheric nitrogen deposition has been found to suppress ANF but much uncertainty still exists. We evaluated the effects of nitrogen addition on ANF in soil, litter, and moss at two topographic positions, that is, valley and slope of a subtropical secondary forest in southwest China. We find that ANF in soil was inhibited by nitrogen addition in the valley but not on the slope, whereas ANF in litter was suppressed by nitrogen addition irrespective of topographic positions. ANF in moss was not affected by nitrogen addition. Our results imply that the responses of ANF to nitrogen addition depend on both topographic position and substrate. Key Points: N2 fixation in soil was inhibited by N addition in the valley but not on the slopeN2 fixation in litter was suppressed by N addition in the valley and on the slopeN2 fixation in moss was not affected by N addition in the valley or on the slope [ABSTRACT FROM AUTHOR]

Free-living N2 fixation is an important pathway of external nitrogen input to natural terrestrial ecosystems. However, few measurements of N2 fixation have been conducted in shrublands. Here, free-living N2 fixation in soil (or soil N2 fixation) and litter (or litter N2 fixation) in three shrublands was measured in a karst catchment, southwest China. The three shrublands were dominated by Pterolobium punctatum Hemsl., Vitex negundo Linn. and Rhus chinensis Mill., respectively. Field measurements were carried out in January and July 2016, respectively, using acetylene reduction assay. N2 fixation had distinct patterns among shrublands or between seasons. In January, no difference was found for soil N2 fixation among the three shrublands, but litter N2 fixation rate was highest in R. chinensis and lowest in P. punctatum. In July, soil N2 fixation rate was highest in R. chinensis and lowest in P. punctatum, but litter N2 fixation was significantly lower in P. punctatum than in the other two shrublands. Across the two seasons, soil N2 fixation rate in R. chinensis was 31% greater than that in P. punctatum, and litter N2 fixation rates in V. negundo and R. chinensis were 13 and 16 times greater than the rate in P. punctatum, respectively. Both soil and litter N2 fixation rates were similar between the two seasons in P. punctatum, but the rates were significantly higher in July than in January in the other two shrublands. Annual N2 fixation rates were estimated to vary from 0.63 ± 0.07 to 0.97 ± 0.08 kg N ha−1 y−1 for the three shrublands. The strongest explanatory variable for soil N2 fixation was total nitrogen in July; and that for litter N2 fixation was nitrogen in January, but was C:N ratio in July. Our findings suggest that large variation in N2 fixation may occur among shrublands within a small scale, and hence, more measurements are needed to get a representative range of N2 fixation rates for the shrubland biome. [ABSTRACT FROM AUTHOR]

Diazotrophic bacteria can reduce N2 into plant-available ammonium (NH4+), promoting plant growth and reducing nitrogen (N) fertilizer requirements. However, there are few systematic studies on the effects of diazotrophic bacteria on biological N2 fixation (BNF) contribution rate and host plant N uptake and metabolism. In this study, the interactions of the diazotrophic Paenibacillus beijingensis BJ-18 with wheat, maize, and cucumber were investigated when it was inoculated to these plant seedlings grown in both low N and high N soils, with un-inoculated plants as controls. This study showed that GFP-tagged P. beijingensis BJ-18 colonized inside and outside seedlings, forming rhizospheric and endophytic colonies in roots, stems, and leaves. The numbers of this bacterium in the inoculated plants depended on soil N levels. Under low N, inoculation significantly increased shoot dry weight (wheat 86.1%, maize 46.6%, and cucumber 103.6%) and root dry weight (wheat 46.0%, maize 47.5%, and cucumber 20.3%). The 15N-isotope-enrichment experiment indicated that plant seedlings derived 12.9–36.4% N from BNF. The transcript levels of nifH in the inoculated plants were 0.75–1.61 folds higher in low N soil than those in high N soil. Inoculation enhanced NH4+ and nitrate (NO3-) uptake from soil especially under low N. The total N in the inoculated plants were increased by 49.1–92.3% under low N and by 13–15.5% under high N. Inoculation enhanced activities of glutamine synthetase (GS) and nitrate reductase (NR) in plants, especially under low N. The expression levels of N uptake and N metabolism genes: AMT (ammonium transporter), NRT (nitrate transporter), NiR (nitrite reductase), NR, GS and GOGAT (glutamate synthase) in the inoculated plants grown under low N were up-regulated 1.5–91.9 folds, but they were not obviously changed under high N. Taken together, P. beijingensis BJ-18 was an effective, endophytic and diazotrophic bacterium. This bacterium contributed to plants with fixed N2, promoted plant growth and N uptake, and enhanced gene expression and enzyme activities involved in N uptake and assimilation in plants. However, these positive effects on plants were regulated by soil N status. This study might provide insight into the interactions of plants with beneficial associative and endophytic diazotrophic bacteria. [ABSTRACT FROM AUTHOR]

Abstract Brazilian scientists have played a pioneering role in developing and applying stable isotope methodologies, in terms of natural abundance and enriched levels, to trace carbon and nutrient flows in terrestrial ecosystems, including natural and agroecosystems. Significant contributions have been made in the areas of biological N2 fixation, carbon dynamics in soil, synthesis and evaluation of labeled fertilizers, and food science. These contributions have originated from several decentralized units of Embrapa, from research institutions, and from federal or state universities. In order to capitalize the existing Brazilian expertise, it is necessary to provide, at an institutional level, analytical facilities for stable isotope research, aiming to strengthen national capacity and to maintain the international competitiveness of the research.

Incorporation of winter legume crops in rotation with rice and using plant growth-promoting rhizobacteria can be a proper alternative approach in increasing sustainable crop production in rice fields. A split factorial field experiment using randomized complete block design with three replications was conducted at the Rice Research Station of Tonekabon, Mazandaran province, Iran, in 2014 to evaluate the effects of previous crop, Azospirillum lipoferum inoculation, and N rate on growth and seed yield of rice (Oryza sativa L. cv. Shiroudi). Main plots were consisted of previous crop [berseem clover, faba bean, and control (fallow)] and subplots of Azospirillum lipoferum (Inoculated and Un-inoculated) and recommended rate of N applications (50, 75, and 100 kg.ha-1). Analysis of variance showed that rice paddy yield was significantly affected by previous crop, Azospirillum lipoferum and N rate. Result also showed that rice paddy yield was increased only by 3% when rice was planted after berseem clover as it compared with rice plant after fallow. However, rice paddy yield was significantly reduced by 16% when it was planted after faba bean as compared to that it was planted after fallow. Rice paddy yield was significantly increased by 14% after Azospirillum lipoferum inoculation. Rice paddy yield was significantly increased by 11% when N application increased from 50 to 75 kg N ha-1, and further N application (100 kg N ha-1) did not affect paddy yield significantly. Based on the result of this experiment, planting rice after berseem clover, Azospirillum lipoferum inoculation and application of kg N ha-1 of recommended rates can be used to obtain highest paddy yield in the experimental site.

Tomorrow’s agriculture, challenged by increasing global demand for food, scarcity of arable lands, and resources alongside multiple environment pressures, needs to be managed smartly through sustainable and eco-efficient approaches. Modern agriculture has to be more productive, sustainable, and environmentally friendly. While macronutrients such as nitrogen (N), phosphorus (P), potassium (K), and sulfur (S) supplied by mineral fertilizers are vital to crop production, agriculturally beneficial microorganisms may also contribute directly (i.e., biological N2 fixation, P solubilization, and phytohormone production, etc.) or indirectly (i.e., antimicrobial compounds biosynthesis and elicitation of induced systemic resistance, etc.) to crop improvement and fertilizers efficiency. Microbial-based bioformulations that increase plant performance are greatly needed, and in particular bioformulations that exhibit complementary and synergistic effects with mineral fertilization. Such an integrated soil fertility management strategy has been demonstrated through several controlled and non-controlled experiments, but more efforts have to be made in order to thoroughly understand the multiple functions of beneficial microorganisms within the soil microbial community itself and in interaction with plants and mineral resources. In fact, the combined usage of microbial [i.e., beneficial microorganisms: N2-fixing (NF), P-solubilizing, and P mobilizing, etc.] and mineral resources is an emerging research area that aims to design and develop efficient microbial formulations which are highly compatible with mineral inputs, with positive impacts on both crops and environment. This novel approach is likely to be of a global interest, especially in most N- and P-deficient agro-ecosystems. In this review, we report on the importance of NF bacteria and P solubilizing/mobilizing microbes as well as their interactions with mineral P fertilization in improving crop productivity and fertilizers efficiency. In addition, we shed light on the interactive and synergistic effects that may occur within multi-trophic interactions involving those two microbial groups and positive consequences on plant mineral uptake, crop productivity, and resiliency to environmental constraints. Improving use of mineral nutrients is a must to securing higher yield and productivity in a sustainable manner, therefore continuously designing, developing and testing innovative integrated plant nutrient management systems based on relevant biological resources (crops and microorganisms) is highly required.

Tomorrow’s agriculture, challenged by increasing global demand for food, scarcity of arable lands, and resources alongside multiple environment pressures, needs to be managed smartly through sustainable and eco-efficient approaches. Modern agriculture has to be more productive, sustainable, and environmentally friendly. While macronutrients such as nitrogen (N), phosphorus (P), potassium (K), and sulfur (S) supplied by mineral fertilizers are vital to crop production, agriculturally beneficial microorganisms may also contribute directly (i.e., biological N2 fixation, P solubilization, and phytohormone production, etc.) or indirectly (i.e., antimicrobial compounds biosynthesis and elicitation of induced systemic resistance, etc.) to crop improvement and fertilizers efficiency. Microbial-based bioformulations that increase plant performance are greatly needed, and in particular bioformulations that exhibit complementary and synergistic effects with mineral fertilization. Such an integrated soil fertility management strategy has been demonstrated through several controlled and non-controlled experiments, but more efforts have to be made in order to thoroughly understand the multiple functions of beneficial microorganisms within the soil microbial community itself and in interaction with plants and mineral resources. In fact, the combined usage of microbial [i.e., beneficial microorganisms: N2-fixing (NF), P-solubilizing, and P mobilizing, etc.] and mineral resources is an emerging research area that aims to design and develop efficient microbial formulations which are highly compatible with mineral inputs, with positive impacts on both crops and environment. This novel approach is likely to be of a global interest, especially in most N- and P-deficient agro-ecosystems. In this review, we report on the importance of NF bacteria and P solubilizing/mobilizing microbes as well as their interactions with mineral P fertilization in improving crop productivity and fertilizers efficiency. In addition, we shed light on the interactive and synergistic effects that may occur within multi-trophic interactions involving those two microbial groups and positive consequences on plant mineral uptake, crop productivity, and resiliency to environmental constraints. Improving use of mineral nutrients is a must to securing higher yield and productivity in a sustainable manner, therefore continuously designing, developing and testing innovative integrated plant nutrient management systems based on relevant biological resources (crops and microorganisms) is highly required. [ABSTRACT FROM AUTHOR]

Quantitative estimates of BNF are needed to improve our understanding of the ecology of N in the environment and aid efforts to improve agricultural N management. Static models based on the principle of 15N isotope dilution have been proposed to estimate the proportion of N in a N2-fixing species that is derived from the atmosphere via biological N2 fixation. Furthermore, equations have been developed to quantify the movement of biologically fixed N between neighboring species or from legumes to cereals in crop rotations. The present paper is structured to provide a comprehensive overview of these methods in a logical and systematic manner. While the relevant literature is vast, some aspects have fortunately been covered by recent in-depth reviews which will be identified and briefly summarized. The overview will emphasize the more practical indirect methodologies based either on artificial 15N enrichment or 15N depletion, or alternatively on 15N natural abundance. In considering methods used to estimate symbiotic dependence, the major structural division is whether or not a non-N2-fixing reference plant is employed, and approaches taken to remove this source of error are described. Four examples are provided to illustrate the contemporary success of 15N-based methods, one in basic research involving endophytic BNF, and three in applied research involving legume breeding for enhanced BNF, the response of legumes to climate change and biotic and abiotic factors affecting legume symbiotic performance. [ABSTRACT FROM AUTHOR]

ABSTRACT Mining is an important economic activity. However, its impact on environment must be accessed, mainly on relevant processes for their sustainability. The objective of this study was to evaluate the diversity and efficiency of symbiotic nitrogen fixing bacterial communities in soils under different types of vegetation in the Quadrilátero Ferrífero: ironstone outcrops, Atlantic Forest, neotropical savanna, and a rehabilitated area revegetated with grass. Suspensions of soil samples collected under each type of vegetation were made in a saline solution to capture rhizobia communities that were then inoculated on cowpea [Vigna unguiculata (L.) Walp.], which was used as a trap plant. The symbiotic efficiency of the communities was evaluated in a greenhouse experiment and the data obtained were correlated to the chemical and physical properties of the soils under each type of vegetation. At the end of the experiment, the bacteria present in the nodules were isolated to evaluate their diversity. The highest numbers of nodules occurred in the treatment inoculated with soil samples from rehabilitated area revegetated with grass and neotropical savanna vegetation, and the lowest numbers were observed in the treatment inoculated with soil samples from ironstone outcrops and Atlantic Forest. In relation to root dry matter, the treatment inoculated with soil samples from Neotropical savanah was superior to those inoculated with soil samples from the other areas; already, in relation to the shoot dry matter, no significant difference among the treatments was observed. The soil properties with the greatest influence on the microbial communities were Al3+ content, considered as high in the Atlantic Forest and neotropical savanna vegetation, as intermediate in the iron outcrops, and as very low in the rehabilitated area revegetated with grass; organic matter, considered as very high in the ironstone outcrops and neotropical savanna, as high in the Atlantic Forest, and as low in the rehabilitated area revegetated with grass; and the pH, with intermediate acidity level in the rehabilitated area revegetated with grass, high level of acidity in the iron outcrops and neotropical savanna, and very high acidity in the Atlantic Forest. After isolation of the nodules, 380 bacterial strains were obtained and separated into 27 groups by cultural characterization analysis. Genetic diversity was evaluated by the 16S rRNA gene partial sequencing of 156 strains, which identified some bacteria belonging to nitrogen-fixing Leguminosae nodulating bacterial genera (Rhizobium, Bradyrhizobium, Burkholderia, and Cupriavidus), some representative of associative bacteria (Bacillus, Paenibacillus, Herbaspirillum, Pseudomonas, and Agrobacterium), and other genera (Brevibacillus, Novosphingobium, Chitinophaga, Dyella, Acinetobacter, and Stenotrophomonas). The highest genetic diversity of bacteria was found in the rehabilitated area revegetated with grass indicated that it was effective in soil rehabilitation

Efficient N2-fixing Leguminosae nodulating bacteria resistant to As may facilitate plant growth on As-contaminated sites. In order to identify bacteria holding these features, 24 strains were isolated from nodules of the trap species Crotalaria spectabilis (12) and Stizolobium aterrimum (12) growing on an As-contaminated gold mine site. 16S rRNA gene sequencing revealed that most of the strains belonged to the group of α-Proteobacteria, being representatives of the genera Bradyrhizobium, Rhizobium, Inquilinus, Labrys, Bosea, Starkeya, and Methylobacterium. Strains of the first four genera showed symbiotic efficiency with their original host, and demonstrated in vitro specific plant-growth-promoting (PGP) traits (production of organic acids, indole-3-acetic-acid and siderophores, 1-aminocyclopropane-1-carboxylate deaminase activity, and Ca3(PO4)2 solubilization), and increased resistance to As, Zn, and Cd. In addition, these strains and some type and reference rhizobia strains exhibited a wide resistance spectrum to β-lactam antibiotics. Both intrinsic PGP abilities and multi-element resistance of rhizobia are promising for exploiting the symbiosis with different legume plants on trace-element-contaminated soils. [ABSTRACT FROM AUTHOR]

Plants on contaminated mining soils often show a reduced growth due to nutrient depletion as well as trace elements (TEs) toxicity. Since those conditions threat plant's survival, plant growth-promoting rhizobacteria (PGPRs), such as rhizobia, might be of crucial importance for plant colonization on TE-contaminated soils. Native rhizobia from mining soils are promising candidates for bioaugmented phytoremediation of those soils as they are adapted to the specific conditions. In this work, rhizobia from Zn- and Cd-contaminated mining soils werein vitroscreened for their PGP features [organic acids, indole-3-acetic acid (IAA), and siderophore (SID) production; 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity; and Ca3(PO4)2solubilization] and Zn and Cd tolerance. In addition, some type and reference rhizobia strains were included in the study as well. Thein vitroscreening indicated that rhizobia and other native genera have great potential for phytoremediation purposes, by exerting, besides biological N2fixation, other plant growth-promoting traits.Leucaena leucocephala–Mesorhizobiumsp. (UFLA 01-765) showed multielement tolerance and an efficient symbiosis on contaminated soil, decreasing the activities of antioxidative enzymes in shoots. This symbiosis is a promising combination for phytostabilization. [ABSTRACT FROM PUBLISHER]

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The complementarity effect (CE) that leads to more efficient utilization of resources has been invoked to explain the biodiversity-ecosystem functioning (BEF) relationships. Dinitrogen (N 2) fixation by legumes is a widely-accepted pathway to enhance N acquisition in diverse species communities. However, the relationships among positive CE, N 2 fixation, and soil N cycling are still poorly understood, especially in agroecosystems. This study tested the hypothesis that positive CE enhances nitrogen (N) acquisition, driven by increased N 2 fixation of legumes and enhanced soil N cycling in intercropping systems. We used a long-term (12 years) field experiment of maize intercropped with both legumes (faba bean, chickpea, and soybean) and a non-legume (oilseed rape) under three phosphorus application rates and a meta-analysis, to explore the main processes for a positive CE of N acquisition in intercropping. Intercropping increases grain yield and N acquisition compared with monocultures, with an increase of 35.1% and 28.0% of the average annual yield and N acquisition, respectively. This N acquisition gain (72 kg N ha−1 in field studies and 14 kg N ha−1 in a meta-analysis) was largely due to the complementarity effect (65 kg N ha−1 in field studies and 25 kg N ha−1 in the meta-analysis), which was significantly correlated with enhanced N 2 fixation in legume-based intercropping systems, in both field studies and the meta-analysis. After 12 years, the soil N concentration of intercropping systems was 4.0–6.3% higher than that of monocultures as a result of enhanced N 2 fixation and N-acquiring enzyme activities, which accounted for positive complementarity in both legume-based and non-legume-based systems. However, those benefits were only observed in later years. We provide a novel mechanism that a positive feedback effect of crop diversity on soil fertility enhances CE over time. Our study demonstrates that enhanced CE is related to N 2 fixation by legumes and a positive feedback effect of crop diversity on soil fertility which highlights that increasing soil N cycling aided by crop diversity may strengthen BEF relationships and enhance grain yield on a decadal scale. These findings have important implications that introducing legumes into managed ecosystems (e.g., cropland, pastures, or agroforestry) or selecting species combinations with different functional groups when designing crop diversity ecosystems are promising approaches to establishing productive and sustainable managed ecosystems. [Display omitted] Enhanced N acquisition by intercropping is mainly due to the complementarity effect. Complementarity effect derives from increased N 2 fixation of intercropped legumes. Complementarity effect is also associated with enhanced soil enzyme activities. Increases of soil N in intercropping are found after a decade, not in early years. New evidences of establishing sustainable ecosystems by using plant diversity. [ABSTRACT FROM AUTHOR]

Seeds of common bean (Phaseolus vulgaris) with high molybdenum (Mo) concentration can supply Mo plant demands, but to date no studies have concomitantly evaluated the effects of Mo-enriched seeds on plants inoculated with rhizobia or treated with N fertilizer. This work evaluated the effects of seed Mo on growth and N acquisition of bean plants fertilized either by symbiotic N or mineral N, by measuring the activities of nitrogenase and nitrate reductase and the contribution of biological N2 fixation at different growth stages. Seeds enriched or not with Mo were sown with two N sources (inoculated with rhizobia or fertilized with N), in pots with 10 kg of soil. In experiment 1, an additional treatment consisted of Mo-enriched seeds with Mo applied to the soil. In experiment 2, the contribution of N2 fixation was estimated by 15N isotope dilution. Common bean plants grown from seeds with high Mo concentration flowered one day earlier. Seeds with high Mo concentration increased the leaf area, shoot mass and N accumulation, with both N sources. The absence of effects of Mo application to the soil indicated that Mo contents of Mo-enriched seeds were sufficient for plant growth. Seeds enriched with Mo increased nitrogenase activity at the vegetative stage of inoculated plants, and nitrate reductase activity at late growth stages with both N sources. The contribution of N2 fixation was 17 and 61 % in plants originating from low- or high-Mo seeds, respectively. The results demonstrate the benefits of sowing Mo-enriched seeds on growth and N nutrition of bean plants inoculated with rhizobia or fertilized with mineral N fertilizer.Sementes de feijoeiro (Phaseolus vulgaris) com alto teor de molibdênio (Mo) podem suprir as demandas vegetais; entretanto, estudos prévios não avaliaram concomitantemente os efeitos do Mo da semente em plantas inoculadas com rizóbio ou sob fertilização nitrogenada. Este trabalho avaliou os efeitos do Mo da semente, no crescimento e na aquisição de N de plantas de feijoeiro, sob N simbiótico ou N mineral, pela mensuração das atividades da nitrogenase e da nitrato redutase e da contribuição da fixação biológica de N2, em diferentes estádios de crescimento. Sementes enriquecidas ou não com Mo foram semeadas sob duas fontes de N, inoculadas com rizóbio ou fertilização nitrogenada, em vasos com 10 kg de solo. No experimento 1, um tratamento adicional consistiu de sementes enriquecidas com Mo e aplicação de Mo ao solo. No experimento 2, a contribuição da fixação biológica de N2 foi estimada pela diluição isotópica de 15N. Plantas de feijoeiro originadas de sementes com alto teor de Mo floresceram um dia antes. Sementes com alto teor de Mo aumentaram a área foliar, a massa de parte aérea e a acumulação de N, nas duas fontes de N. A ausência de efeitos do Mo adicional aplicado ao solo indicou que as sementes enriquecidas com esse nutriente foram suficientes para o crescimento vegetal. Sementes enriquecidas com Mo elevaram a atividade da nitrogenase, no estádio vegetativo de plantas inoculadas, e da nitrato redutase, em estádios tardios nas duas fontes de N. A contribuição da fixação biológica de N2 foi de 17 e 61 %, em plantas oriundas de sementes com baixo e alto teor de Mo, respectivamente. Os resultados evidenciaram os benefícios das sementes enriquecidas com Mo para o crescimento e a nutrição nitrogenada do feijoeiro, quando inoculado com rizóbio ou sob adubação nitrogenada.